During muscle development, changes in gene transcription occur during specific phases of the differentiative process. In the proposed experiments a labeled muscle-specific complementary DNA (cDNA) will be used as a hybridization probe to obtain quantitative information regarding the regulation of expression of muscle-specific genes during myogenesis in vivo. Analysis of the total poly A+RNA population of myoblasts by cDNA hybridization indicates that the bulk of this class of RNA represents transcriptions from "housekeeping genes" which are expressed in all cells. A small, but measurable fraction of the cDNA to myoblast poly A+RNA is complementary to RNA transcripts which are probably present only in developing muscle. This muscle-specific cDNA will be isolated by preparative hybridization techniques designed to eliminate cDNA complementary to RNA from a non-myogenic tissue (embryonic brain). Extensive tests will indicate whether the cDNA which fails to hybridize to brain RNA is indeed "muscle-specific". The isolated muscle-specific cDNA will be used to estimate the diversity of the muscle-specific genes as well as determining the intracellular localization and numbers of copies per myoblast cell of the transcripts from muscle-specific genes. The results of these experiments will provide important information regarding the role and mechanisms of the expression of phenotype-specific gene sets during molecular and embryonic differentiation of myogenic cells are well as eucaryotic cells in general.